Absorption of Water by Plants

Major portion of water required by plants is absorbed by roots but in some cases water may be absorbed by leaves and stems also. Root hair is a specially modified epidermal cell meant for absorption of capillary water of the soil.

The plasma membrane and the vacuolar membrane (tonoplast) act as semipermeable membranes and water is absorbed by osmosis. Soil solution should have a higher water potential as compared to root hair cell, then only water will enter the root hair cell. Once into the root hair, water will pass into cortical cells, endodermis, pericycle and into the xylem vessel.

The movement of water is purely dependent on water potential gradient. Water movement into the plant follows two pathways - symplast and apoplast.

Cytoplasm of the entire plant is connected through plasmodesmata which are the protoplasmic strands forming the symplast system. Water movement through the cells take this symplast pathway by osmosis.

The cell wall and the intercellular spaces form the apoplast pathway which allows water movement inside the plant by the phenomenon of capillarity and adsorption.

The water absorbed through the roots is transferred radially to the xylem, from where it reaches to all the other parts of the plant body by vertical conduction of water through the xylem vessels. The content of xylem vessels is known as xylem sap.

Root Pressure Theory

If a stem is cut few inches above from the ground with a sharp knife, xylem sap is seen flowing out through the cut end. This phenomenon is known as exudation and this is due to the positive pressure developed within the root system due to continuous absorption of water by osmosis which develops a positive pressure known as root pressure. This pressure can be measured and ranges from 3 to 5 atmospheres. But this pressure is enough to raise water to small heights in herbaceous plants, but it does not explain rise of water in stems of tall trees that are taller measuring 10 to 100 meters.

Physical Force Theory or Cohesion Theory

This theory takes into account the physical forces which explain uplift of water to great heights in very tall trees. The three forces that act together are force of cohesion (attraction between water molecules), force of adhesion (attraction between water and lignocellulose walls of xylem) and transpiration pull which lifts the water column by creating a tension inside the xylem vessel.

Water forms an unbroken column starting from the intercellular space of the leaf mesophyll to the xylem of the leaf, through stem and root to the water in the soil. A water potential gradient exists between the leaf to the root and transpiration causes a pull of the entire water column. So long as the column is an unbroken one from the outer atmosphere, through the plant upto the soil, water is lifted up by the force of transpiration pull.